Heart of Darkness is clearly a work of prose fiction, and it is also somewhat autobiographical, as pointed out by the previous answerer.

It should be noted that the language of Heart of Darkness is rich in emotion, in metaphor, in simile, in rhythm, in imagery,  in aural beauty -- that is, in all the elements that are usually emphasized in poetry. 

So, although it would be a stretch to call H.O.D. a poem, it would be correct to say that its prose is poetic.

Joseph Conrad’s Heart of Darkness is one of the author’s most famous works of prose fiction. It was first published in 1899 and it is considered a fictional autobiography since he based it on his actual experience in the African Congo. Conrad was sent up the river to rescue an agent at a remote station for the company he worked for at the time.

In the story, Conrad is Charlie Marlow, a seaman on board a boat to rescue a mysterious man named Kurtz. It is on one level about a voyage into the heart of the Belgian Congo, and on another about the journey into the soul of man.

The book was used as a basis for the movie Apocalypse Now, but instead of going up a river in the Belgian Congo during the late nineteenth century, the movie takes place on a river in Viet Nam, during the war there in the twentieth century.