Three-dimensional models of heat transfer have been established and numerically solved using a commercial software package, Fluent, in order to obtain distributions of temperature, velocity, pressure, and liquid volume fraction of the polymer. The influences of the boundary conditions on the phase change of the polymer and the temperature distribution in the die have been evaluated. The results show that the temperature of the region close to the pelletizing surface is relatively low due to the cooling effect of the cool water, while the temperature deeper inside the die is higher, with a lower temperature gradient, as a result of the heating effect of the hot thermal oil and the polymer. A solidification phase change of the polymer occurs near the polymer outlet due to heat loss from the polymer to the water, while deeper inside the hole the polymer remains fluid without solidification, due to heating by the thermal oil. Numerical simulation provides a reliable method to optimize the design of the die, the choice of metallic material for the die, and the operating conditions of the polymer pelletizing under water.