The book "The Whisperer of the City of Chimneys" by Nishino Akihiro

The city of chimneys is surrounded by a 4,000 meter high wall. There are so many chimneys in the city that the air is constantly polluted, and none of the inhabitants have ever seen a blue sky. On Halloween night, a courier loses the heart he was supposed to deliver to its owner. He can't find the heart... The heart continues to beat where it was lost - in the city of chimneys.