This study aims to develop a model to predict the formation of compounds during the supercritical water gasification of biomass with a thermodynamic equilibrium approach. A Gibbs free energy minimization routine has been developed using Mat Lab software to predict the equilibrium state compounds of the system. Regarding all the calculations, pure condensed phases (both liquids and solids), gases, and aqueous compounds (both neutral and ionic species) have been taken into account in the subcritical region, and in the supercritical region, aqueous hydrate complexes have been taken into account instead of the aqueous compounds. The model has been validated for subcritical and supercritical regions separately. A case study for supercritical water gasification of a microalgae feedstock sample has been performed. The effects of temperature, pressure, and dry matter content on the phase behavior of elements and main product gases have also been investigated. The results of this work show that the model developed on the basis of Gibbs free energy minimization involving multiphase compounds can be used to predict the formation of equilibrium state compounds during supercritical water gasification of biomass.