The grand partition function for the two-dimensional, two-component plasma at Г = 2 is evaluated exactly in a finite system for various interfaces: a charged hard wall (the so-called primitive electrode model), a second two-component plasma of different fugacity separated by an impermeable membrane (the ideally polarizable interface), and a metal wall separated by an impermeable barrier. For each of these models the surface tension is calculated directly from the asymptotic expansion of the grand partition function.