A two-dimensional model of methane thermal decomposition reactors is developed which accounts for coupled radiative heat and polydisperse carbon particle nucleation, growth, and transport. The model uses the NavierStokes equations for the fluid dynamics, the radiative transfer equation for methane and particle species radiation absorption, the advectiondiffusion equation for gas and particle species transport, and a sectional method for particle species nucleation, heterogenous growth, and coagulation. The model is applied to a tubular laminar flow reactor. The simulation results indicate the development of a reaction boundary layer inside the reactor, which results in significant variation of the local particle size distribution across the reactor. (C) 2011 American Institute of Chemical Engineers AIChE J, 58: 25452556, 2012