In the present study, a lumped capacitance, 0-D model was developed for simulating thermal runaway of Li-ion battery cells. The model accounts for multi-mode heat transfer, physics associated with gas generation, evaporation of liquid electrolyte, and venting of gases out of the cell during thermal runaway. The model was exercised for two different cases of oven test and external heater. It was found that the role of evaporative cooling changes depending on the state of the decomposition reactions at the time of vent-activation. For oven tests with low temperature and for external heating at slow rates, the energy budget is delicately balanced between decomposition reactions and heat exchange with the surroundings. In these scenarios, the evaporative cooling has a significant effect, and the characteristic temperature decrease after venting is observed. Under faster heating scenarios, vent activation occurs at a time when the decomposition reactions are underway. The evaporative cooling effect is less significant in these scenarios, and the temperature vs. time signature does not show the characteristic temperature decrease. The model presented in this work provides a useful tool for parameter identification, sensitivity analysis, and probing the effects of gas generation, evaporation, and venting.