We present a novel, simple method for predicting the operating conditions under which overheating by gas-phase exothermic olefin polymerization may lead to melting. The boundaries of the operating regions which can lead to melting are constructed using a simple, pseudo-steady, lumped thermal model which ignores temperature gradients within the particle and assumes that the active-sites distribution is uniform within growing polymer particles. It is shown that intraparticle temperature rise has a negligible impact on the boundaries of the safe region. The boundary of the unsafe operating region of the simple model is adequate also for predicting melting by dynamic models, such as the multilayer and polymeric-flow models. The region of safe operation can be increased by decreasing the initial particle diameter or the loading of the catalyst. The predicted boundaries of safe operating conditions are somewhat conservative as the simple model does not account for the time during which the particle heats to attain a pseudo steady state with the ambient gas. The particle Lewis number has a strong impact on the initial temperature rise.