Ethyne, which has the formula C2H2, is a nonpolar molecule. Ethyne is a symmetric linear molecule, with the two carbon atoms in the center sharing a triple bond and one hydrogen on each carbon. Since ethyne is a symmetric linear molecule, the electronegativities of all its constituent atoms cancel out.
For a molecule to be polar, there must be a net charge. This requires that one part of the molecule be more electronegative than the rest of the molecule, and that it not be balanced elsewhere in the molecule. For ethyne, the electronegativities of the carbons cancel out, and the electronegativities of the hydrogens cancel out. The molecule is therefore nonpolar.