The conditional expectation of the product of the first $n-1$ Hermite polynomials in a multivariate normal distribution with respect to the $n$-th variable. A fresh perspective on the Kibble-Slepian formula

We calculate the conditional expectation of $\prod_{j=1}^{n-1}H_{k_{j}}% (X_{j})$ given $X_{n}=z,$ if random vector $(X_{1},\ldots,X_{n})^{T}$ has multivariate normal distribution and $H_{n}(x)$ denotes $n$-th Hermite polynomial. This expectation is a polynomial in $z$ of order $\sum_{j=1}% ^{n-1}k_