Flammability limit and combustion severity are vital and indispensable in process loss prevention. Combustion of gaseous C2H4/O-2, C2H4/air, CH4/O-2, and CH4/air mixtures at various concentrations with initial conditions of 1 bara and 273 K were experimentally investigated in a 20 L apparatus. The two objectives of this study were (1) to originally predict the gaseous mixture flammability limit by the calculated adiabatic flame temperature (CAFT) and to analyze CAFT influential factors and (2) to creatively predict the experimental gas explosion pressure by the equilibrium-calculated combustion pressure values. The CAFT values and equilibrium pressure were obtained by a chemical equilibrium model. The results revealed that the CAFT criteria of 1530 and 1230 K effectively predicted the CH4 and C2H4 flammability zones. Moreover, selecting the adiabatic flame temperature corresponding to the upper flammable limit of O-2 in the fuel as a CAFT criterion effectively predicted the flammability zone (as verified by the experimental and reference values for simple hydrocarbons). The experimental and equilibrium-calculated pressure values indicated that the combustion along the stoichiometric line increased linearly with an increasing C2H4 concentration (>5.0 vol %). Moreover, the equilibrium-calculated pressure is always higher than the experimental value considering loss in energy. In short, this study can help predict the flammability zone and combustion severity for a flammable gas, while the measurements are unlikely to be implemented.