As a successive work to clarify the film growth kinetics of GaAs, a new lumped model for computer simulation of film growth during MOCVD process is proposed. The film growth process in a horizontal reactor using trimethylgallium (TMGa) and tertiarybutylarsine (TBAs) is considered. Analyzing methodologies at two different scales have been used to revise our previous simulation model. One is the micro-scale analysis using a selective-area growth method and the other is the reactor-scale macro analysis using numerical simulation. Species concentration profiles are obtained from the macro-scale computational fluid dynamics study incorporated with chemical reactions. Surface reaction rate constants are extracted from the selective-area growth experiments. From the multi-scale analysis that combined the two-scale analyses, surface formation from the source gas, TMGa, is included to the previous model. The kinetic data for the surface reaction of TMGa are deduced from the quantitative analysis for the species concentrations from both of the analyses. Simulation results using the new model show that monomethylgallium is the major source of film deposition, but deposition from TMGa is important in the upstream region of the reactor.