A number of potential runaway reaction systems of the fine chemical industry (e.g., polymerization processes) undergo significant changes of their physical properties with the reaction extent, resulting in thickening phenomena of the reaction mass and in a consequent decay of the reactor heat-transfer efficiency. In this work, set of key process indicators have been developed, allowing for monitoring of safe operating conditions of industrial semibatch polymerization reactors without requiring any kinetic characterization of the reaction system. The only required information are the heat of reaction and some fully available process variables that are normally recorded for any industrial reactor. The proposed criterion has been successfully validated using some experimental data measured on an industrial polymerization reactor for the production of polyacrylic rheology modifiers.