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This paper describes the results of numerical simulation of second-harmonic waveguide free-electron lasers (FELs) from the small-signal regime to the large-signal regime. Aimed at reducing the size and hence the cost of compact waveguide FELs operated from the microwave to the far infrared, these unconventional waveguide FELs can substantially decrease the minimum electron energy required for strong FEL radiation at a given frequency while increasing the small-signal gain. This contribution focuses on their saturation behaviors, taking into consideration variation in wiggler field and electron-energy spread. Depending on the roundtrip power loss within the FEL cavity and the initial electron-energy spread, the computed relationship between interaction gain and in-cavity power can be used to maximize the output power at a given electron current. Furthermore, it is found that gain degradation due to electron-energy spread remains relatively unchanged regardless of radiation power and wiggler field.