1) Look at the target block and type "/chimney tgt"...or stand on the target block and type "/chimney std".
2) They will then emit smoke particles, simulating a Chimney.
I'm removing Chimneys soon, as they're buggy, cause slight lag, and aren't as good as I thought.
*In the normal world*: Hellhounds, Aggressive Zombie Pigmen, Winter Wolves, Giants
*In the Nether*: Fire Zombies, Fire Spiders, Fire Giants, Fire Slimes, Fire Skeletons...and pretty much Fire Everything Else.
*In the Skyland*: Creepers that throw you instead of blow up, Lightning Creepers.