A model for the growth of gallium nitride in a vertical metallorganic vapor phase epitaxy (MOVPE) reactor is presented and compared to experimental growth rate measurements. For a mixture of nondilute gases, the flow, temperature, and concentration profiles are predicted using recent kinetic data. Growth rates are predicted based on simple reaction mechanisms and compared with those obtained experimentally These comparative results show that the growth of GaN epi layers proceeds through an intermediate adduct of trimethylgallium and ammonia. Loss of adduct species due to oligmerization leads to the lowering of the growth rate. Quantification of this loss of reacting species is made based on experimentally observed growth rates. An apparent chemistry model is presented based on the salient features of GaN MOVPE. Process conditions are perturbed to obtain trends in growth rate and uniformity in order to demonstrate the utility of such a model in optimizing the GaN MOVPE process.