Of the large number of equations of state available in the literature, few treat small molecules as well as polymers, association, and copolymers. Three equations of state that rigorously account for all of these effects and have been applied to real polymer systems are the lattice-fluid theory (LF), the statistical associating fluid theory (SAFT), and the perturbed hard-sphere chain theory (PHSC). We illustrate the versatility of one of these equations of state (SAFT) for modelling the phase behavior of polyolefin systems, and the effects due to molecular dissimilarity, compressibility, association, and copolymer composition.