Applicability of the equation of state models for modeling and simulation of phase equilibrium in polymer production processes is investigated. The flash separation of unreacted ethylene from polyethylene is used as a prototype for simulation. First, three equation of state models (statistical associating fluid theory, Sanchez-Lacombe, and an augmented Soave-Redlich-Kwong cubic equation of state) are comparatively used to correlate pure component volumetric, calorimetric, and phase equilibrium properties for ethylene and polyethylene. Next, they are tested for correlation of vapor-liquid equilibrium of ethylene + low-density polyethylene mixtures. Finally, a two-stage flash problem that mimics the separation process in the low-density polyethylene production is simulated using the three models. Each equation of state model has some unique characteristics that affect the outcome of modeling pure component as well as mixture behavior. The advantages and disadvantages of each model are discussed.