Nitrogen rejection processes are usually needed for two natural gas sources: sub-quality natural gas reserves and produced gas from enhanced oil/gas recovery technologies. The nitrogen content of natural gas in the former is usually constant during the project lifetime, but it varies from 5 to 70% during enhanced oil/gas recovery programs. This variation leads to different process flowsheets for nitrogen removal: single-column, double-column, three-column, and two-column processes. In order to determine which configuration is more suitable for a particular nitrogen content in a feed stream, we must minimize the energy requirement for each process. In this study, we merge all the four configurations into two categories: single-column and multi-column processes and then use the Particle Swarm Optimization algorithm to optimize process parameters for each process with the objective of energy consumption minimization. Finally, we use the exergy concept to analyze theoretically these different processes. (C) 2018 Elsevier Ltd. All rights reserved.