  A certain representation for the Heisenberg algebra in finite-difference
operators is established. The Lie-algebraic procedure of discretization of
differential equations with isospectral property is proposed. Using
$sl_2$-algebra based approach, (quasi)-exactly-solvable finite-difference
equations are described. It is shown that the operators having the Hahn,
Charlier and Meixner polynomials as the eigenfunctions are reproduced in
present approach as some particular cases. A discrete version of the classical
orthogonal polynomials (like Hermite, Laguerre, Legendre and Jacobi ones) is
introduced.
