We present a numerical method of conformal mappings of unbounded multiply-connected domains exterior to closed Jordan curves onto the three types of canonical domains of Nehari, i.e., the parallel slit domain, the circular slit domain and the radial slit domain. In the method, we express the mapping functions in terms of a pair of conjugate harmonic functions and approximate them, using the charge simulation method, by a linear combination of complex logarithmic functions. The method is simple without integration and suited for domains with curved boundaries. In particular, approximate mapping functions of an unbounded multiply-connected domain onto the three types of slit domains are obtained in a unified way by solving linear equations with a common coefficient matrix. A typical example shows the effectiveness of the method.