An approximate solution is presented to the phase equilibrium problem for the case of a partially miscible polymer solution or a binary heteropolymer blend. It is based on the Flory-Huggins theory and is valid for mixtures well into the phase-separated region. Analytical expressions are derived for the extent and composition of each of the two coexisting phases as a function of the component molecular size and interaction strength. These results are combined with Helfand's model to provide estimates for the interfacial tension and thickness and, therefore, a measure of the quality of the interfacial bonding in a polymer/polymer blend. They are also utilized in conjunction with a free volume theory approach for calculating the mutual diffusion coefficient of a penetrant, migrating through a polymer.