Yeah, if you are familiar with harmonic balance theory, envelope simulation principles are the analogous - like having harmonic balance with time varying coefitients - this time variations corresponds to the LF modulation of a HF carrier. It is a kind of generalization of HB simulations. I tried to run one in ADS, it was an awfull experience. If you don't know exactly what you are doing do'nt do it! There are a number of options that can influence your results significantly, you have to have theoretical background on this and this is quite a long story.
FlyHigh