A method for monitoring a polymer resin-producing polymerization reaction in a fluid bed reactor, including by determining in on-line fashion a maximum diluent (e.g., ICA) concentration and an optimal diluent (e.g., ICA) concentration in the reactor, whereby performing the reaction with diluent (e.g., ICA) concentration less than the maximum diluent concentration ensures an acceptably low risk that the resin in the reactor in the presence of condensable diluent gas will reach a condition of limiting stickiness. Preferably, the optimal diluent concentration maximizes production rate subject to relevant constraints. The method can also include at least one of the steps of controlling the reaction to achieve a desired production rate by controlling diluent (e.g., ICA) concentration, and controlling the reaction to prevent reactor temperature from reaching a reduced melt reference temperature at least substantially equal to a dry melt reference temperature minus a temperature depression value by which the dry melt reference temperature is depressed by presence of at least one condensable diluent gas with the resin in the reactor.