TECHNOLOGY FOCUS - POLYMERS - Preferred Properties: The per pass conversion of alpha-olefin to polyolefin in the reactor is less than 5%.Preferred Composition: In the feed stream, the gas replacing the ballast gas comprises 75-95% ethylene, 25-2.2 wt.% ethane and 0-2.5 wt.% non-polymerizable 3-5C gas(es) condensable at 20-50 degrees C at the pressures of the heat exchanger for the recycle stream. The gas replacing the ballast gas comprises 10-50% of the gas in the feed stream.Preferred Components: The 4-6C copolymerizable alpha-olefin is butene and/or hexene. The stream replacing the ballast stream is a stream from an ethylene cracker, which has only been treated to remove the methane and the hydrocarbons having more than 6C.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Catalyst: The catalyst is selected from Ziegler Natta catalysts and transition metal complexes containing at least one 5-13C ligand (preferably a cyclopentadienyl radical, an indenyl radical or a fluorenyl radical) containing at least one unsaturated cyclic moiety pi-bonded to the transition metal, the ligand being unsubstituted or further substituted by halogen atom(s) and/or 1-4C alkyl radical(s). The transition metal complex is activated with an aluminoxane or an activator capable of ionizing the complex.Preferred Activator: The activator is an aluminoxane of formula R42AlO(R4AlO)mAlR42 (where R4 is 1-4 hydrocarbyl and m is 5-30) and is present in an amount to provide a molar ratio of aluminum to transition metal from 50:1 to 1000:1. Alternatively, the activator is:(i) compounds of formula (R5)+(B(R7)4)- (where R5 is a cyclic 5-7C aromatic cation or a triphenyl methyl cation and R7 is phenyl optionally substituted with 3-5 of F, 1-4C alkyl or alkoxy radical which is optionally substituted by F; or a silyl radical of formula -Si(R9)3, where R9 is H or 1-4C alkyl);(ii) compounds of formula ((R8)tZH)+(B(R7)4)- (where Z is N or P, t is 2 or 3 and R8 is 1-8C alkyl, phenyl optionally substituted by up to three 1-4C alkyl radicals, or one R8 taken together with the N atom forms an anilinium radical) or(iii) compounds of formula B(R7)3. EXAMPLE - In a computer model, the feed stream comprised 26746 kg/hour ethylene, 1511 kg/hour comonomer, 0.973 kg/hour hydrogen and 1542 kg/hour nitrogen. The vent off recycle stream was 153 kg/hour. A stream of purge gas to the polymer discharge system was 1324 kg/hour. The monomer recovery unit had an output of 1944 kg/hour of gas, which could be returned to the reactor or vented to flare. Liquids at 1775 kg/hour were sent back to the reactor. The output of polymer was 29550 kg/hour. The computer simulation was re-run using a condensed mode and replacing all of the ballast gas (N2) with ethane and also running in dry mode (no condensable gas) and replacing the ballast gas with ethane. At condensed and dry mode rates, production rates were 35727 and 21552, respectively, for simulation with ethane replacing nitrogen while simulation with nitrogen at condensed and dry mode rates exhibited production rates of 29550 and 14252, respectively.