Reaction kinetics of Powder River Basin coal gasification were measured in a modified drop tube fixed bed reactor, accompanied with a rapid response, real-time gas analysis system. The material was rapidly heated and quickly pyrolyzed, followed by a slow gasification with long residence time. Tests are done at 833 degrees C and elevated pressure to 4 atm. The conditions were chosen to minimize mass transfer restrictions, so that only surface reactions were measured. The experimental data was fitted using Langmuir-Hinshelwood models to describe both reaction kinetics and degree of surface saturation. CO2 addition into steam gasification, steam addition into CO2 gasification, and injection of steam into an in-progress CO2 gasification test were performed. We found a rarely observed competition mechanism, in which H2O molecules primarily occupy active sites when H2O and CO2 are both present. With sufficient steam partial pressure, the reaction rate in mixtures of H2O and CO2 is equal to steam reaction rate. A newly developed L-H type model was presented to describe this competition mechanism. This model contains a steam reaction rate term and a supplementary CO2 reaction rate term, and matches our experimental results. Published by Elsevier Ltd.