While classic reforming processes rely on heat and chemical equilibrium, plasma-based reforming processes possess the ability to induce non-equilibrium and reactive chemistry at low temperatures using high energy electrons. To better understand the distinctive roles of both electron-induced chemistry and thermochemistry during plasma-assisted fuel reforming, we previously developed a temperature-controlled dielectric barrier discharge (DBD) reactor, which controlled the gas temperature and the electron temperature independently. Here, we investigate plasma-assisted steam reforming of methane using the temperature-controlled DBD reactor and electron-kinetics calculations. We investigated the individual effects of the determining factors for electron-induced chemistry (i.e., reduced electric field intensity and discharge power) and for thermochemistry (i.e., background gas temperature) by varying the discharge power, gas temperature, and pressure inside the reactor. As a result, we found that both the electron-induced chemistry and thermochemistry governed the reactant conversions. Thermochemistry positively affected the methane conversion in particular, but negatively affected the water conversion as the gas temperature increased. The electron-induced chemistry weakly affected the product distribution, while the background temperature (thermochemistry) strongly influenced the product selectivity and composition by altering the chemical pathways involving the plasma-generated reactive species at the given temperature.