1. A method for monitoring a polymer resin-producing polymerization reaction in a fluid bed reactor to determine a current value of a stickiness control temperature, including the steps of: (a) in on-line fashion, generating acoustic data indicative of at least one value of an acoustic condition in the reactor; and (b) in on-line fashion, determining a current value of the stickiness control temperature from the acoustic data and a predetermined relation between values of the acoustic condition in the reactor and values of the critical temperature, where the stickiness control temperature is a temperature indicative of a characteristic of melting behavior of polymer resin in reactor.