A tubular packed bed reactor for the steam-CO2 combined reforming of natural gas to produce the synthesis gas of a target H2/CO ratio 2.0 was simulated. The effects of the reactor dimension, the feed gas composition, and the gas feeding temperature upon the possibility of coke formation across the catalyst bed were investigated. For this purpose, 2-dimensional heterogeneous reactor model was used to determine the local gas concentrations and temperatures over the catalyst bed. The thermodynamic potential distribution of coke formation was determined by comparing the extent of reaction with the equilibrium constant given by the reaction, CH4+2CO ⇔3C+2H2O. The simulation showed that catalysts packed in the central region nearer the entrance of the reactor were more prone to coking because of the regional characteristics of lower temperature, lower concentration of H2O, and higher concentration of CO. With the higher feeding temperature, the feed gas composition of the increased H2O and correspondingly decreased CO2, or the decrease in the reactor diameter, the volume fraction of the catalyst bed subsequent to coking could be diminished. Throughout the simulation, reactor dimension and reaction condition for coking-free operation were suggested.