Over-heating of semitransparent fused silica (quartz) pieces within MetalOrganic Chemical Vapor Deposition (MOCVD) reactors may result in parasitic deposition on reactor walls, leading to loss of precursors. Although growth on the substrate (epitaxial growth) is diffusion-limited, parasitic deposition occurs at colder temperatures and is therefore, rate-limited. The modeling of low-temperature deposition requires complex chemical mechanisms, which account not only for the kinetics of decomposition, but also the kinetics of adsorption and desorption at the surfaces. In this article, the role of complex chemistry has been demonstrated for growth of Gallium-Arsenide in a commercial horizontal reactor (Crystal Specialties 425).Numerical computations were performed for a wide range of operating conditions. Comparison of numerical predictions with experimental data clearly indicates the need for the development and use of detailed chemistry in modeling parasitic deposition in commercial reactors.