The massive potential in biomass gasification could satisfy the rising energy demand. A promising technology for sustainable hydrogen production is supercritical water gasification of biomass (SCWG). This study proposes a new model to assess gas yields and char formation through SCWG. To this end, a thermodynamic approach is utilized to model the reactor, assuming the equilibrium condition. The impact of catalyst on the SCWG is also involved in the new model, considering a deviation term for the Gibbs free energy of solid char. Two different feedstocks, including sunflower and corncob, are assessed toward SCWG. The newly developed model considerably improves gas yields and char formation predictions considering the experimental data. Compared to the non-modified modeling strategy, the sunflower and corncob's gas yield and char formation are improved by 85.37 and 62.52, respectively. The sensitivity results indicate that temperature and feed concentration substantially impact the gas yields and char formation, while pressure is less impactful.