A unified thermodynamic modeling approach for feedstock gasification under adiabatic conditions at the carbon boundary point (CBP) using O-2, steam, or/and CO2 as gasification agents is proposed by considering the total hydrogen/carbon ratio, total oxygen/carbon ratio, and total enthalpy entering into the gasifier per mole of carbon as the unifying variables. Three approaches, namely, iterative graphical, noniterative graphical, and analytical approaches, are proposed to predict the amount of a gasification agent required to reach CBP, for any feedstock composition with any combination of gasification agents entering at any inlet temperature. The unified model is validated for 20 fuels chosen on the boundary of the extended Van Krevelen diagram, with the results obtained using Aspen Plus. The model predicts the amount of the gasification agent within an error of 10%. The unified model is also used to predict the gasification temperature, syngas composition, and cold gas efficiency within an error of 10%.