The paper presents basic data on Enhanced Gas Recovery (EGR) by gas-gas displacement for nearly depleted natural gas reservoirs by injecting waste gases. The soundness of the concept of gas-gas displacement for enhancing gas recovery was investigated via laboratory investigations, compositional modelling and economic analyses. Paramount Resources is field testing the concept in their GRIPE Project in the Athabasca region of Alberta, to enhance production from a gas bearing stratum overlying the oil sand interval. This paper is a part of a series of papers presenting results of EGR research conducted over a five-year period (2003-2007). The main targets were volumetric (closed) reservoirs in advanced phases of exploitation. Results of basic research on methane displaced from core samples by pure gases (pure CO2 or pure N-2), as well as flue gases (mixtures of CO2 and N-2), are presented. A series of nine gas/gas displacement tests in 30 cm long, 4 cm diameter, Berea cores were conducted at a temperature of 70 degrees C and a pressure of 6,200 kPa. Most of the tests were conducted in the presence of connate water, while others were conducted without connate water (dry cores) to confirm and benchmark the results by other investigators. The tests on consolidated cores showed that for pure nitrogen and pure CO2, used as the displacing medium, the recovery was comparable. Where a mixture of CO2 and nitrogen displaced the natural gas, it was observed that there was a delay in CO2 breakthrough, associated with a period when only a mixture of methane and nitrogen was produced. This is so because Solubility of CO2 in connate water is considerably higher than that of nitrogen. This leads directly to a higher gas recovery due to a longer exploitation period, given the fact that Lip to 20% nitrogen can be tolerated in the produced stream, as opposed to only 1% for the CO2 case. For this period of methane and nitrogen production, there are no operational problems associated with the corrosive nature of CO2. EGR by gas-gas displacement is seen as a promising way of prolonging the productive life and economic recovery of many depleting volumetric gas reservoirs.