Due to the increasing price and decreasing amount of fossil fuels, energy obtained from biomass has received much attention in recent years. Biomass generally contains much water and they must be dried first to be used in conventional thermochemical gasification process. However, a large amount of energy is consumed when the biomass are dried. In order to avoid the drying process, hydrothermal gasification processes, like as supercritical water gasification (SCWG , has attracted more attention because water can be used as a reaction medium, so that wet biomass d es not need in the dried to be processed. The SCWG is related to present large hydrogen yields, but the excess of water inhibits the carbon monoxide production in the system and thus the syngas production is very low. The use of co-reactants appears as astrategy to promote syngas production in SCWG processes. In this work optimization techniques are applied in the minimization of Gibbs energy and in entropy maximization to study the thermodynamic effect of the use of CH4 and CO2 as co-reactant in the SCWG of microalgae biomass (Nannochloropsis sp.) and sugarcane bagasse. The Virial equation of state was used to represent the non-ideality of the systems. Both problems were formulated as optimization problems (non-linear programing) and the software GAMS in combination with the CONOPT solver, were used to solve them. Syngas with a H2/CO molar ratio close to 2 (ideal for Fischer-Tropsch synthesis applications) was obtained with an addition of CO2 as a co-reactant. The calculated final temperatures were close to the initial temperature of the reaction in both systems, thus indicating low energy requirements for maintain these reactions.