A nonlinear study of the efficiency, power, and the saturation length of a free-electron laser with realistic helical wiggler and ion-channel guiding using numerical simulation is presented. A set of coupled nonlinear differential equations is derived using the Lorentz force equation for the ensemble of electrons and Maxwell’s equations for radiation waves. To calculate the efficiency, power, and the saturation length of the amplifier, these equations are solved simultaneously using a code developed in this study. By choosing different values of the plasma frequency of ion-channel guiding, the dominant mode for the group I and group II orbits is found. The effect of ion-channel guiding on efficiency and saturation length was studied, and the results show that ion-channel guiding increases efficiency and reduces saturation length. Moreover, the efficiency increases, and the saturation length decrease when the annular electron beam is substituted for a solid electron beam. Finally, the results show that the efficiency of a realistic wiggler free-electron laser is higher than that of an idealized one, while the saturation length is lower.