\begin{filecontents}{mytestbib.bib}
@book{goossens93,
author = "Frank Mittelbach and Michel Goossens and Johannes Braams and David Carlisle and Chris Rowley",
title = "The {LaTeX} Companion",
year = "1993",
publisher = "Addison-Wesley",
address = "Reading, Massachusetts"
}
\end{filecontents}
\documentclass{article}
\usepackage{filecontents}
\usepackage{natbib}
\usepackage{bibentry}
\nobibliography*
\begin{document}
A full in-text cite of \bibentry{goossens93}.
A regular citation of \cite{goossens93}.
\bibliographystyle{plainnat}
\bibliography{mytestbib}
\end{document}