This paper deals with a method for the numerical solution of parabolic initial-boundary value problems in two-dimensional polygonal domains~$\Omega$ which are allowed to be non-convex. The Nitsche finite element method (as a mortar method) is applied for the discretization in space, i.e. non-matching meshes are used.
For the discretization in time, the backward Euler method is employed.
The rate of convergence in some $H^1$-like norm and in the $L_2$-norm is proved for the semi-discrete as well as for the fully discrete problem.
In order to improve the accuracy of the method in presence of singularities arising in case of non-convex domains, meshes with local grading near the reentrant corner are employed for the Nitsche finite element method. Numerical results illustrate the approach and confirm the theoretically expected convergence rates.