Symbolism, Imagery, Allegory
While Levi never describes The Chimney, it is mentioned several times by other prisoners, and in very ominous tones. It is usually mentioned in terms of a prisoner being "ready for the Chimney." Fancy Literary Term Alert! "Chimney" here is a synecdoche for the crematoria, the giant ovens in which the dead bodies of Jews were burned. Warning: pretty disturbing image to contemplate. This means that it's being used as a figure of speech where the part ("Chimney") signifies the whole ("crematorium"). Since this is the case, it's quite clear that The Chimney is the central symbol for death in the book.
Why do the prisoners use "Chimney" rather than just straight up referring to the crematoria? Think about it: would you want to be reminded on a daily basis of the horrible fate that most likely awaited you? Or would you try to find some way to talk around that issue to maybe not go stark raving mad? Saying "The Chimney" is a strategy for distancing oneself from the horrifying reality that almost most of these prisoners would eventually face.