The same CAE model used for the filling and packing stage in the gas-assisted injection molding (GAIM) process simulation was also applied to simulate the cooling phase. This was made possible by using the line source method for modeling cooling channels. The cycle-averaged and cyclic transient mold cavity surface temperature distribution within a steady cycle was calculated using the three-dimensional modified boundary element technique similar to that used in conventional injection molding. The analysis results for GAIM plates of a semicircular gas channel design attached with a top rib are illustrated and discussed. It was found that the difference in cycle-averaged mold wall temperatures may be as high as 10 degrees C, and within a steady cycle, part temperatures may also vary by about 15 degrees C. The conversion of the gas channel into equivalent circular pipe and further simplification into two-node elements using the line source method not only affects the mold wall temperature calculation very slightly but also reduces the computer time by 93%. This indicates that it is feasible to achieve an integrated process simulation for GAIM under one CAE model, resulting in great computational efficiency for industrial application.