In-text citation: what is the correct way of using both last name (i.e., \citeauthor ) and typical citation (i.e., \cite) I am using plain citation style with biblatex and was wondering about the convention of using \citeauthor (and probably \citeyear) along with \cite 

While writing sometimes I do the following: 


  \citeauthor{johnTKDE} (\citeyear{johnTKDE}) proposed a method for
  ...etc


should I also use \cite at the end of the sentence when using \citeauthor ? what is the convention of using all these together? (Computer Science if the field matters).
I don't think there is something like a general convention for this, aside from the usual rule of being consistent. So follow your personal/conference/journal/university/whatever style.

If that style dictates to use numbered references such as [1], always include this. Even if, like in your example, you start one of your sentences with the author name (and year). Note that (as @Ian_Fin mentions in the comments) the year is usually omitted when using an author name with number styled references.