Globally, large amounts of biomass wastes such as cattle manure, fruit/vegetable waste, and cheese whey residual streams are disposed of from farming and food processing industries. A promising approach to convert such biogenic residues into valuable biofuels is Supercritical Water Gasification (SCWG). A detailed investigation on SCWG of the mentioned wet biomass wastes has been performed to assess the thermodynamic behavior of such a complicated system. This is conducted by combining advanced models with a supplementary experimental study, providing deep insight into the behavior of the SCWG system for different bio-waste sources. For the modelling part, different approaches including global, constrained and thermal quasi-thermodynamic equilibria have been pursued to analyze the influence of operating parameters on the produced biogas quality. Furthermore, SCWG experiments were conducted using biomass samples provided by our industrial partner. Reasonable agreements were observed between experimental results and predictions from constrained and thermal-quasi equilibrium models, showing significant improvements over the global thermodynamic equilibrium model. Results showed that superimposition of carbon conversion efficiency together with the use of a constant molar amount of specific compounds can improve the accuracy of the global equilibrium model. Furthermore, comparisons between different models revealed the advantage of the thermal quasi-equilibrium model, which uses the “approach temperature” concept, over the constrained equilibrium model, by reducing the complexities inherent in superimposing multiple constraints. Overall, the thermal-quasi equilibrium approach has its advantages of lumping all the additional constraints used in the constrained equilibrium model into an effective approach temperature, offering (i) a better reproducibility of the experimental data point and (ii) a rigorous basis for scale-up calculation. The results of this study provide a better understanding of the SCWG process for different types of wet biomass feedstocks as result of applying advanced analytical approaches and comparing with experiments.