A novel approach for determining vapor-liquid equilibrium (VLE) for polymer-diluent systems is described. The concentration- and temperature-dependence of diluent-polymer Flory-Huggins interaction parameter chi(ij) is first estimated from depression in polymer melting point as a function of monomer (diluent) concentration. VLE for the polymer-diluent binary system is then obtained using an activity coefficient model and equation of state model with Aspen Polymer Plus. The technique is validated for the syndiotactic polystyrene-toluene and syndiotactic polystyrene-methyl ethyl ketone and is extended to polybutylene terephthalate - with various diluents such as butanediol, tetrahydrofuran, isophorone, 1-methylnaphthalene, and benzonitrile, systems hitherto not reported in the literature.