Retrieving an alt tag from a custom field

The following ought to be much closer to what you want to achieve:

<div class="imgHentry">
    <a href="https://wordpress.stackexchange.com/questions/74228/<?php the_permalink(); ?>" class="goto-icon">
        <?php the_post_thumbnail(
            'thumbnail',
            array( 'alt' => get_post_meta( $post->ID, 'alt_tag', true ) )
       ); ?>
    </a>
</div>