The present disclosure is directed to a process for producing olefin-based polymer in a gas phase polymerization reactor. The process includes forming a wet zone in the gas phase polymerization reactor. The wet zone is formed by maintaining a temperature less than or equal to the fluidizing medium dew point temperature+2° C. in a region of the reactor. The region is defined as the region extending from the distributor plate to 2.5 meters above the distributor plate. Injection of a high activity catalyst composition in the wet zone produces olefin-based having a settled bulk density greater than 23.5 lb/ft3.