Despite many research works concerning the decomposition and/or gasification reaction pathways for the biomass constituent compounds and on developing lumped reaction mechanisms for real biomass samples, a complete and detailed reaction network for a real biomass has not been proposed to date. Based on the published literature data, this study aims to develop an integrated decomposition and gasification kinetic model using Aspen Plus software for cellulose, hemicellulose, lignin, and protein model compounds in sub- and supercritical water to model the supercritical water gasification of wet biomass feedstocks regarding the carbon gasification efficiency and gas yields. The model involves 55 reactions in the subcritical region and 74 reactions in the supercritical region. The validation of the model by comparing the experimental results of others has been performed, and the results of the model were found to be in an agreement. The model is capable of predicting reliable results for temperatures up to 650 degrees C, at pressures between 25-30 MPa, and at dry matter concentrations up to 10 wt % in the feedstock. In addition, case studies for microalgae, pig-cow manure mixture, and paper pulp gasification in supercritical water have also been performed.