Scattering experiments were conducted on a critical, binary mixture of polyethylene (PE) and head-to-head polypropylene (PP), and a symmetric PE-PP diblock copolymer melt. The binary mixture exhibited a liquid-liquid phase transition at 143 degrees C, while the block copolymer exhibited an order-disorder transition at 149. degrees C. Estimates of the Flory-Huggins interaction parameter between PE and PP, and the statistical segment lengths of the chains were obtained by fitting neutron scattering profiles from the PE/PP blends and the PE-PP block copolymer, in the single-phase regime, to theoretical predictions based on the random phase approximation (RPA). These estimates were used to compute the scattering profiles of multicomponent blends comprising PE, PP, and PE-PP. A phase diagram for these blends was constructed on the basis of the computed scattering profiles. In the paper that follows this one, we compare the theoretical computations with experimental data from multicomponent PE/PP/PE-PP blends.