Interactions between reaction rates, conduction, and diffusion inside catalyst particles can be complex, especially when influenced by nonuniform surface conditions produced by the flow field external to the particle, or by the highly directional temperature field near a heated tube wall. In this work, a three-dimensional, realistic flow field was coupled to species and energy balances in cylindrical catalyst particles using computational fluid dynamics (CM). Two strongly endothermic reactions were studied: methane steam reforming and propane dehydrogenation. Detailed pellet surface and intraparticle temperature, species, and reaction rate distributions were obtained for a near-wall particle. Nonuniform and nonsymmetric surface and intraparticle variations were observed. These effects are primarily attributed to the steep temperature gradients at the tube wall, as well as depletion of the reactants in regions of low or stagnant flow where particles approach each other closely.