Steam-methane reforming (SW) reaction was studied using a tubular reactor packed with NiO/gamma-Al(2)O(3) catalyst to obtain synthesis gases with H(2)/CO ratios optimal for the production of synthetic diesel fuel from steam-hydrogasification of carbonaceous materials. Pure CH(4) and CH(4)-CO(2) mixtures were used as reactants in the presence of steam. SMR runs were conducted at various operation parameters. Increasing temperature from 873 to 1,023 K decreased H CO ratio from 20 to 12. H(2)/CO ratio decreased from 16 to 12 with pressure decreasing from 12.8 to 1.7 bars. H(2)/CO ratio also decreased from about 11 to 7 with steam/CH(4) ratio of feed decreasing from 5 to 2, the lowest limit to avoid severe coking. With pure CH(4) as the feed, H(2)/CO ratio of synthesis gas could not be lowered to the optimal range of 4-5 by adjusting the operation parameters; however, the limitation in optimizing the H(2)/CO ratio for synthetic diesel fuel production Could be removed by introducing CO(2) to CH(4) feed to make CH(4)-CO(2) mixtures. This effect can be primarily attributed to the contributions by CO(2) reforming of CH(4) as well as reverse water-gas shift reaction, which led to lower H(2)/CO ratio for the synthesis gas. A simulation technique, ASPEN Plus, was applied to verify the consistency between experimental data and Simulation results. The model satisfactorily Simulated changes of H(2)/CO ratio versus the operation parameters as well as the effect of CO(2) addition to CH(4) feed.