\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}