This paper presents a methodology for analyzing the heat-transfer process during the injection molding of plastics as an aid to mold design. A numerical scheme using the Boundary Element Method (BEM) with “zonal” approach has been developed to solve the quasi-steady temperature field and its normal derivative over the entire surface of the mold plates including the cavity wall as well as parting surface. In order to obtain a solution for the temperature field, a cycle-averaged heat-transfer coefficient is introduced from a transient heat-conduction analysis and applied as the boundary condition at the cavity surface. The numerical predictions as compared with the experimental data have shown that the cycle-averaged solution used in this study gives a reasonable representation of the transient temperature variation over the cavity surface. Based on the numerical predictions, the mold designer will be able to design a proper cooling-system for a mold to achieve better part quality and high productivity through more uniform cooling and shorter cycle time, respectively.