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This paper presents the underlying method for developing a computationally-efficient simulation code for the ultrasonic leaky Lamb-wave imaging technique. This technique allows for effective imaging through a highly contrasting (steel) cylindrical layer with immersion transducers and in particular, for detection of interfaces located beyond the steel layer and evaluation of the mechanical properties of the material bonded to the steel layer. The cw transmitting and receiving transducers are modeled via the complex-transducer-point method which accounts for eccentering of the cylindrical structure housing the transducers. The imaged interface may be eccentered with respect to the casing. To treat this case within the canonical analytical framework, we apply a simple and approximate high-frequency Kirchhoff-based spectral wavenumber-shifting scheme to the contribution to the receiver voltage emanating from reflections at this interface. Comparisons against finite difference results show that this simple scheme captures well the eccentering effects, on the amplitude of the reflection echo.