Olefin polymerization in fluidized-bed reactors may lead to particle overheating. The resulting softening or melting of polymer particles generates polymer sheets, requiring reactor shutdown. Previous criteria predicting the operating conditions leading to this overheating accounted only for the propagation reaction. We determine here the impact of accounting also for the polymerization initiation and deactivation reactions on the safe region of operation, i.e., one in which the maximum transient particle temperature does not reach the melting temperature. The polymeric-flow model predicts a slightly smaller region of safe operation than a lumped-thermal model and uniformly distributed catalytic sites. The reason is that the polymeric-flow model predicts a slightly higher maximum temperature under the same operating conditions. In general, accounting for the initiation and deactivation steps increases the region of safe operation. A decrease of either the initiation or propagation rate increases the size of the safe operating region, while decreasing the deactivation rate decreases its size. The deactivation impact is important mainly when its rate is not much slower than that of initiation. Parametric sensitivity does not occur before melting for typical gas-phase olefin polymerization operation. Thus, this potential problem need not be considered.