Die Röstgase verlassen den Trommelröster.

Here Die means the, not she.

It is the plural definite article for Röstgase:

• das Röstgas = the roasting gas / a roasting gas
• die Röstgase = the roasting gases

So Die Röstgase means the roasting gases.

German die can mean different things depending on context:

• die Frau = the woman
• die Gase = the gases
• die = she / they in some other contexts

In this sentence, it is clearly the article belonging to Röstgase.

Because the noun is Gase, which is the plural of Gas.

• das Gas = gas
• die Gase = gases

So:

• das Röstgas = roasting gas
• die Röstgase = roasting gases

The verb also shows that the subject is plural:

• verlassen = leave
  This is the plural form here, matching die Röstgase.

If the subject were singular, you would get:

• Das Röstgas verlässt den Trommelröster.

Röstgase is a compound noun, which is very common in German.

It is made from:

• rösten = to roast
• Gas / Gase = gas / gases

So Röstgase literally means something like roasting gases or gases produced during roasting.

In technical German, compounds are often used where English might use a longer phrase.

Trommelröster is also a compound noun:

• die Trommel = drum
• der Röster = roaster

So der Trommelröster means the drum roaster.

As in many German compounds, the last part determines the gender:

• der Röster is masculine
• therefore der Trommelröster is also masculine

Because Trommelröster is the direct object of the verb verlassen, and direct objects take the accusative case.

The dictionary form is:

• der Trommelröster = nominative

But in the accusative:

• den Trommelröster

This change happens because the noun is masculine.

So the sentence structure is:

• Die Röstgase = subject
• verlassen = verb
• den Trommelröster = direct object

A helpful comparison:

• Der Trommelröster ist heiß.
  Here it is the subject, so nominative: der
• Die Röstgase verlassen den Trommelröster.
  Here it is the object, so accusative: den

Because verlassen usually takes a direct object, not a preposition.

So German says:

• den Trommelröster verlassen = to leave the drum roaster

This is similar to English leave the room, not necessarily leave from the room.

You could express the idea differently with another verb and a prepositional phrase, for example:

• Die Röstgase treten aus dem Trommelröster aus.
  = The roasting gases come out of the drum roaster.

But with verlassen, the normal structure is:

• something verlassen
• not usually aus etwas verlassen

Here verlassen means leave in the sense of exit.

So:

• Die Röstgase verlassen den Trommelröster.
  = The roasting gases leave / exit the drum roaster.

But verlassen can also mean abandon in other contexts:

• Er verlässt seine Familie.
  = He abandons his family.

So the exact English translation depends on context. In this technical sentence, leave or exit is the natural meaning.

Because this is a normal main clause in German, and in German main clauses the finite verb usually comes in second position.

So the pattern is:

• Subject + verb + object

Here:

• Die Röstgase = subject
• verlassen = verb
• den Trommelröster = object

This is the most neutral word order in German.

Yes, English might also say simply roasting gases, depending on style and context.

German often uses the definite article where English may or may not use one. So:

• Die Röstgase verlassen den Trommelröster.

could be translated as:

• The roasting gases leave the drum roaster.
• Roasting gases leave the drum roaster.

The best choice depends on the surrounding text. In technical writing, both can be possible.

In German compounds, the last part usually decides the gender.

So:

• das Gas → das Röstgas
• der Röster → der Trommelröster

That is a very useful rule for learners.

Examples:

• das Haus → das Wohnhaus
• der Tisch → der Küchentisch
• die Maschine → die Kaffeemaschine

So when you see a long compound noun, look at the final element first.

Because in German, all nouns are capitalized.

So in this sentence:

• Röstgase is a noun
• Trommelröster is a noun

That is why they begin with capital letters.

This is a standard rule in German spelling and helps you recognize nouns more easily.

It would be:

• Das Röstgas verlässt den Trommelröster.

Changes:

• Die Röstgase → Das Röstgas
• verlassen → verlässt

So the plural sentence is:

• Die Röstgase verlassen den Trommelröster.

and the singular version is:

• Das Röstgas verlässt den Trommelröster.

This is a useful way to see how the article and verb change with number.

It sounds technical or industry-specific, not everyday conversational German.

Words like:

• Röstgase
• Trommelröster

belong to a specialized context such as roasting technology, coffee production, food processing, or industrial equipment.

Grammatically, though, the sentence is completely normal and useful for learning:

• plural subject
• verb in second position
• accusative masculine object
• compound nouns

So even if the vocabulary is technical, the grammar is very typical German.