In this paper, a multivariate Nakagami-m distribution is derived using Royen's gamma distributions of one-factorial accompanying matrices for a constant correlation model. The cumulative distribution function (CDF), the probability density function (PDF) and the covariances are obtained in infinite series form. From these results, we derive outage probabilities of selection combiners (SC) in both interference-limited and noise-limited scenarios with a constant correlation model over Nakagami-m fading assuming arbitrary average powers at each input of the combiner.