Snake modules introduced by Mukhin and Young form a family of modules of quantum affine algebras. The aim of this paper is to prove that the Hernandez-Leclerc conjecture about monoidal categorifications of cluster algebras is true for prime snake modules of types A n and B n . We prove that prime snake modules are real. We introduce S-systems consisting of equations satisfied by the q-characters of prime snake modules of types A n and B n . Moreover, we show that every equation in the S-system of type A n (respectively, B n ) corresponds to a mutation in the cluster algebra A (respectively, A ′ ) constructed by Hernandez and Leclerc and every prime snake module of type A n (respectively, B n ) corresponds to some cluster variable in A (respectively, A ′ ). In particular, this proves that the Hernandez-Leclerc conjecture is true for all prime snake modules of types A n and B n .