We report the activity and the selectivity of several heterogeneous nickel catalysts for the supercritical water gasification (SCWG) of biomass. The effects of catalyst support on the carbon conversion and hydrogen selectivity were demonstrated using 44 different materials, covering a wide range of chemical and physical properties. At 5% nickel loading, α-Al2O3, carbon nanotubes (CNTs), and MgO supports resulted in high carbon conversions, while SiO2, Y2O3, hydrotalcite, yttria-stabilized zirconia (YSZ), and TiO2 showed modest activities. Utilization of different γ-Al2O3 supports resulted in a wide range of catalytic activities from almost inactive to highly active. Other catalyst carriers such as zeolites, molecular sieves, CeO2, and ZrO2 had insignificant activity under the conditions tested (i.e., 380 °C, 2 wt% feed). Aside from the catalytic activity, the stable metal oxide supports under the experimental conditions of this work, as identified by XRD, were α-Al2O3, boehmite, YSZ, and TiO2. Given the high hydrogen yield and carbon conversion as well as its superior stability in supercritical water, α-Al2O3 was chosen for a more elaborate investigation. It was found that when using the same amount of nickel, the methane yield significantly decreased by increasing the nickel to support ratio whereas the carbon conversion was only slightly affected. At a given nickel to support ratio, a threefold increase in methane yield was observed by increasing the temperature from 350 to 410 °C. The catalyst activation conditions (e.g., calcination and reduction) had a small impact on its catalytic performance. The catalyst activity increased with the addition of alkali promoters (i.e., K, Na, Cs) and decreased with the addition of tin. The highest catalytic activity was obtained with the addition of 0.5% potassium. In summary, nickel loading and alkali promoters improved the hydrogen selectivity and the carbon conversion of the Ni/α-Al2O3 catalyst.