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In this paper, we modify the Gaussian noise model (GN-model) to address the nonlinearity effects in few-mode fibers. Closed-form expressions for the nonlinear interference power in birefringent few-mode fibers (FMFs) are derived and the effect of differential mode group delay (DMGD) is investigated. Moreover, the nonlinearity accumulation through propagation in multiple-spans fiber and the birefringence effect are considered. In addition, we discuss the effect of the DMGD on the fiber nonlinearity in systems adopting mode-division multiplexing (MDM). The results show that the DMGD management degrades the system performance in weak coupling regime because the nonlinear interference is enhanced. However, strong coupling-based transmission outperforms weak coupling transmission regardless of the DMGD effect in the weak coupling regime. On the other hand, by taking the DMGD effect into account …