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 on reactor walls 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 parasitic deposition and rate-limited growth has been demonstrated for growth of gallium arsenide in a commercial horizontal MOCVD 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 surface chemistry mechanisms in modeling parasitic rate-limited deposition in order to accurately predict the growth rate on the target surfaces in commercial MOCVD reactors.