A method, including the steps of: (a) providing a polymerization reaction which produces a polymer resin in a fluid bed reactor, wherein the polymer resin is characterized by a dry melt reference temperature that is a characteristic of the melting behavior of a dry version of the polymer resin wherein the dry melt reference temperature is a dry melt initiation temperature determined graphically as the onset of rapid melting in a first or second melt DSC curve, determined from a DSC measurement on a dry sample of the polymer resin as set out in the experimental section; and (b) determining in response to data indicative of at least one monitored parameter of the reaction, in on-line fashion, a reduced melt reference temperature characteristic of the melting behavior of the polymer resin as it exists in the reactor, and wherein the reduced melt reference temperature is a reduced melt initiation temperature, wherein the reduced melt reference temperature is equal to the difference between the dry melt reference temperature and a melt reference temperature depression value, "D", where D is a temperature by which the dry melt reference temperature is depressed by the presence of the diluent with the resin in the reactor wherein the diluent is a condensable gas or a mixture of condensable gases that is present in the polymerization reactor with the polymer resin being produced; and (c) controlling the reaction in response to the reduced melt reference temperature wherein step (a) includes the step of monitoring reactor temperature, at least one of the density and melt index of the polymer resin, and concentration of at least one diluent present with the resin in the reactor, and step (b) includes the steps of: determining said dry melt reference temperature in response to data indicative of at least one of density and melt index of the polymer resin; and determining said temperature depression D by processing data, including data indicative of reactor temperature, concentration of each said diluent present with the resin in the reactor, and said dry melt reference temperature, in accordance with a predetermined model. (a) providing a polymerization reaction which produces a polymer resin in a fluid bed reactor, wherein the polymer resin is characterized by a dry melt reference temperature that is a characteristic of the melting behavior of a dry version of the polymer resin wherein the dry melt reference temperature is a dry melt initiation temperature determined graphically as the onset of rapid melting in a first or second melt DSC curve, determined from a DSC measurement on a dry sample of the polymer resin as set out in the experimental section; and (b) determining in response to data indicative of at least one monitored parameter of the reaction, in on-line fashion, a reduced melt reference temperature characteristic of the melting behavior of the polymer resin as it exists in the reactor, and wherein the reduced melt reference temperature is a reduced melt initiation temperature, wherein the reduced melt reference temperature is equal to the difference between the dry melt reference temperature and a melt reference temperature depression value, "D", where D is a temperature by which the dry melt reference temperature is depressed by the presence of the diluent with the resin in the reactor wherein the diluent is a condensable gas or a mixture of condensable gases that is present in the polymerization reactor with the polymer resin being produced; and (c) controlling the reaction in response to the reduced melt reference temperature wherein step (a) includes the step of monitoring reactor temperature, at least one of the density and melt index of the polymer resin, and concentration of at least one diluent present with the resin in the reactor, and step (b) includes the steps of: determining said dry melt reference temperature in response to data indicative of at least one of density and melt index of the polymer resin; and determining said temperature depression D by processing data, including data indicative of reactor temperature, concentration of each said diluent present with the resin in the reactor, and said dry melt reference temperature, in accordance with a predetermined model. determining said dry melt reference temperature in response to data indicative of at least one of density and melt index of the polymer resin; and determining said temperature depression D by processing data, including data indicative of reactor temperature, concentration of each said diluent present with the resin in the reactor, and said dry melt reference temperature, in accordance with a predetermined model.