Order by a meta field in query loop

<?php
$cat = get_post_meta( get_the_ID(), 'cat_page', true );

$args = array(
    'category_name' => $cat,
    'meta_key' => 'niveau_chiffre',
    'orderby' => 'meta_value_num', //Since we're dealing with numbers
    'order' => 'ASC',
    //'meta_query' => array(
       // array(
           //'key' => 'niveau-chiffre',
           //'value' => 'niveau-chiffre', Don't give a value because we're not looking for a specific one
           //'compare' => '>=', We're not comparing as such. Defaults to =
           //'type' => 'NUMERIC'
        //)
    ), //You missed a comma here I think
);

Try this.