A simple method to predict the liquid-liquid (LL), vapor-liquid (VL), and vapor-liquid-liquid (VLL) boundaries of (solvent + polymer) systems found in the solution process is developed with theoretical justification. The method is developed using the (solvent + LLDPE) data of only two polymers and applied to the (solvent + polymer) data of 276 different systems containing 65 different solvent combinations (up to five solvent constituents) and 69 different nonaromatic hydrocarbon polymers with predominantly linear backbones. For data sets of interest to the solution process, lower critical end point (LCEP) temperatures and pressures can be estimated with average absolute deviations (AADs) of 4 K and 0.2 MPa, respectively. The effects of ethylene and/or comonomer addition and nitrogen addition are accurately predicted (VL/VLL: AAD = 0.2 MPa, n = 1084; LL: AAD = 0.5 MPa, n = 1115). Good results are obtained when extending the results of pure solvents to commercial-grade solvents.