Nickel-containing monoliths can be used to eliminate tar and ammonia in a real biomass gasification gas. They can work with a fuel gas containing important amounts of particulates, as in the case of the fuel gas produced in fluidized-bed gasifiers. The use of monoliths is a very recent and promising technology that has not yet reached a commercial level and requires experimental studies at pilot scale. Those studies indicate that tar and ammonia conversions (eliminations) with these monoliths depend on so many experimental variables that a model is needed to understand and correlate the results obtained with these monolithic reactors. A model is developed in this paper for the monolithic reactor that has two very different zones: the gas reheating zone and the monolith itself. The model is developed according to the basic rules of chemical reaction engineering, and it includes two microkinetic models for the tar and NH3 elimination reactions, mass balances for tar and NH3, and a heat balance in the monolith. Several important facts appear and are confirmed in this study, such as the control of the external mass transfer in the channels of the monolith, which generates a dependence of the overall kinetic constants on the temperature of potential, not Arrhenius, type. The AT across the monolith is also analyzed in detail. The equations developed for the model are easy to handle, allow a correct analysis of experimental data, and may be used to design new monolithic reactors for this application.