A mixture of Nitrogen and hydrocarbon gas was used as a cycling gas in a large rich gas condensate reservoir to maintain reservoir pressure and thus prevent condensate drop-out due to pressure decrease below the dew point pressure. The objective of using Nitrogen gas is to partially replace some of hydrocarbon injection gas, which has a high demand for Abu Dhabi domestic consumption. This paper presents the plan, implementation, results and analysis of an intensive reservoir monitoring program, which was conducted to monitor the Nitrogen injection and production. Additionally, the paper discusses lessons learned from the Nitrogen injection, inferences reached, and corrective actions taken. The Nitrogen injection was started as a Mixed Case (MC) injection, where a mixture of Nitrogen (~20%) and hydrocarbon gas (~80%) was injected in all existing injectors across the entire reservoir for a period of three years. The plan was to stop the MC injection and apply a Pure Case injection (PC), which consists of injecting only pure Nitrogen in the North West area of the reservoir. The objective of the PC injection is to localize the Nitrogen injection to be able to apply Nitrogen removal technology in a specific part of the existing surface facilities whenever high Nitrogen production is reached. Monitoring the Nitrogen production relies mainly on an intensive gas composition sampling and analysis, which have been conducted quarterly for all gas producers, in addition to other surveillance activities of the intensive reservoir monitoring program. The Nitrogen monitoring results and analysis of the MC injection showed Nitrogen breakthrough in many wells with increasing Nitrogen production trends. Different analytical techniques were applied to evaluate and predict the Nitrogen breakthroughs. Due to the unavailability of surface Nitrogen removal facilities and the increase in Nitrogen production during the MC injection, a Nitrogen risk assessment study supported a recommendation to stop the MC injection and continue cycling with dry hydrocarbon gas only as before. Also, it was decided to delay the planned PC injection for more evaluations and studies.