A new reactor system for gas-phase ethylene/alpha-olefin polymerization is described. Good gas-phase temperature control at high polymerization rates was achieved with the 2-L semi-batch reactor. Ethylene/1-hexene and ethylene polymerization results showing the effects of operating conditions on temperature profiles are presented. Good gas-phase temperature control is required to obtain reliable activity profiles. A gas-sampling and analysis system, which allows relatively rapid (< 3 min) and accurate determination of ethylene/1-hexene contents in the gas-phase of the reactor, is also described. Rapid and reliable hydrogen contents were also measured with this relatively inexpensive system.