This paper analyzes the achievable rate of the single-carrier system with a frequency-domain equalizer (SC-FDE) under the receiver in-phase/quadrature phase (I/Q) imbalance in the radio frequency front-end. We consider linear schemes based on the minimum mean square error (MMSE), zero-forcing equalization, and a matched filter. We derive the achievable rate of the SC-FDE system hit by the I/Q imbalance for the mentioned scenarios. We show that depending on the value of I/Q imbalance parameters, the achievable rate of the system approaches a saturation level. To examine the validity of our analysis, we compare it with the simulation results and exhibit their consistency. We also consider nonlinear schemes such as an iterative block decision feedback equalizer (IB-DFE), which is an iterative scheme. We obtain the mutual information achieved by the IB-DFE in the high-signal-to-noise-ratio regime and after enough number of iterations.