Steam-methane reforming (SMR) reaction was studied using a tubular reactor packed with NiO/γ-Al2O3 catalyst to obtain synthesis gases with H2/CO ratios optimal for the production of synthetic diesel fuel from steamhydrogasification of carbonaceous materials. Pure CH4 and CH4-CO2 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 H2/CO ratio from 20 to 12. H2/CO ratio decreased from 16 to 12 with pressure decreasing from 12.8 to 1.7 bars. H2/CO ratio also decreased from about 11 to 7 with steam/CH4 ratio of feed decreasing from 5 to 2, the lowest limit to avoid severe coking. With pure CH4 as the feed, H2/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 H2/CO ratio for synthetic diesel fuel production could be removed by introducing CO2 to CH4 feed to make CH4-CO2 mixtures. This effect can be primarily attributed to the contributions by CO2 reforming of CH4 as well as reverse water-gas shift reaction, which led to lower H2/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 H2/CO ratio versus the operation parameters as well as the effect of CO2 addition to CH4 feed.