To make steam
On any prop you choose (you will make it invisible when done), move it to where you will want the steam to originate and add script "attach emitter." The following settings are a good start, but then make trial-and-error adjustments and see what looks best to you:

texture file: smoke3a.png
part. life:60
emitter life: 1000 
particles / sec: 10 
speed: 30 
start scale: 5 
end scale: 2 
%fading in: 90
%fading out: 5
enable fade turned on
alpha scale: 0.05
spawn cube: x:1200 y:60 z:1200 
normal: x:1 y:0 z:1

To hold down lag, don't let the steam go into or through a floor, ceiling, or wall.

then make the prop invisible