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In fiber-optic wavelength-division multiplexing (WDM) communications systems, the nonlinearity of the channel and the limitations of optical filters result in received channel signals that are correlated. Furthermore, the square-law nature of the detector converts the additive fiber amplifier noise to a data-dependent asymmetric and non-Gaussian noise. The optimal multiuser detector for a Gaussian approximation is compared to the actual statistics and found to give a close performance estimate. Through bounds on the error probability and the asymptotic multiuser efficiency, the optimal multiuser detector based on a Gaussian approximation is found to improve significantly performance in highly correlated data cases. This result is applied to a practical optical WDM system.