The Soave-Redlich-Kwong (SRK) and the Sanchez and Lacombe (SL) equations of State are applied to the flash simulation of polyethylene industrial separators, specifically low-density polyethylene (eight resins.) and linear low-density polyethylene (25 resins). Three mixing rules are used in the SRK equation: van der Waals (VDW) one-fluid, Wong-Sandler, and LCVM (linear combination of the Vidal and Michelsen mixing rules). The latter two mixing rules incorporate the Bogdanic and Vidal activity coefficient model. All these models involve two adjustable parameters. The results indicate that SL is the best model to simulate the flash separation of polyethylene under industrial conditions.