Chemical reaction modeling in a seleniztion process for CIGS solar cell films has been developed. The selenization process is a heat treatment in a hydro-selenide gas which is highly toxic. As temperature increases, selenium is deposited on substrate after decomposition and then reacts with Cu, In, and Ga. Decomposition modeling of hydro-selenide and surface reaction modeling of a CIGS layer is developed to predict physical and chemical phenomena in the selenization process. Turbulent fluid dynamics and multi-mode heat transfer are also considered. Turbulent fluid dynamics and convective heat transfer are simulated using a large-eddy simulation (LES) technique while thermal conduction and radiation are predicted using finite-element methods. The present technique well predicts the distribution of temperature and composition on substrates in both space and time.